1. Technical Field
The present invention relates to a temperature compensation for a piezoelectric oscillator which measures a position based on positioning signals from GPS (Global Positioning System) satellites, and particularly to temperature compensation for a piezoelectric oscillator that is installed in a TSXO (Temperature Sensor Xtal Oscillator) as a piezoelectric oscillator leaving a temperature compensation function to an external unit and provided by an external temperature compensation circuit.
2. Related Art
A receiving device such as a mobile phone unit having a GPS function, a mobile phone device having a GPS receiving function or the like demodulates and analyzes positioning signals transmitted from plural GPS satellites and thus measures the current position. As a reference oscillator used for these receiving devices, a temperature compensated piezoelectric oscillator TCXO (Temperature Compensated Xtal Oscillator) having little frequency change depending on temperatures is broadly used. The reason for this broad use is that as the oscillator with a built-in receiving device has higher frequency accuracy, the search range for catching positioning signals transmitted from GPS satellites can be narrowed and consequently the position can be measured in a shorter time by reducing the search time, that is, by reducing the time to catch positioning signals from GPS satellites.
Meanwhile, at the time of startup of the above receiving device or the like, for example, at the time of turning on power of the device, temperature of the entire device rises in a short time. In the case of a mobile phone or the like, temperature quickly changes when the mobile phone is moved from outdoors to indoors or from indoors to outdoors. Therefore, there is a problem that temperature compensation is unstable until temperature is stabilized within the oscillator. To overcome this problem, it is demanded that a temperature compensation circuit capable of quickly responding to temperature changes is independently constructed on the user side and the oscillator acquires temperature information of a piezoelectric vibrator installed in the oscillator, whereby carrying out appropriate temperature compensation. To cope with this demand, a TSXO which does not require a temperature compensation circuit is applied on the oscillating circuit side, equipped with a temperature sensor which outputs the temperature of the installed piezoelectric vibrator to the user side, and a storage circuit which stores frequency temperature information (temperature coefficient) of the installed piezoelectric vibrator and outputs the frequency temperature information to the user side (as disclosed in JP-A-2003-324318).
In the case of using a crystal vibrator utilizing thickness-shear vibration, an oscillation signal outputted from the oscillator has temperature dependence drawing a positive cubic curve. In a GPS system or the like where the above TSXO is stalled and the user side has a temperature compensation circuit that is connected to the TSXO, the temperature compensation circuit calculates the quantity of temperature compensation and corrects the frequency so that the frequency becomes constant at any temperature, on the basis of temperature information acquired from the temperature sensor and frequency temperature information acquired from the storage circuit.
Here, the frequency temperature information stored in the storage circuit is acquired in the manufacturing and inspecting process. Therefore, in view of throughput at the time of manufacturing, generally, frequency temperature information in the case where temperature changes in either way, that is, at the time of temperature rise or temperature fall, is acquired and stored in the storage circuit.
By the way, the frequency temperature characteristic of the crystal vibrator has a hysteresis characteristic. Having a hysteresis characteristic means that the crystal vibrator has different temperature dependence at the time of temperature rise and at the time of temperature fall. The causes of this difference includes the fact that a change in strain stress of the crystal vibrator in response to a temperature change cannot follow the actual temperature change, and thermal strain changes or the like of the supporting structure, adhesive, deposited alloy, electrodes and the like in the oscillator. This difference becomes more conspicuous for a smaller crystal vibrator.
In the field of high-accuracy electronic devices such as mobile phone terminals equipped with the above GPS function, the tolerance range of frequency deviation (Δf/f0) is very narrow, and for example, the frequency deviation (Δf/f0) within the temperature range from −30° C. to 85° C. needs to be within ±0.5 ppm.
Therefore, with the method in which frequency temperature information in the case where temperature is changed either way, that is, at the time of temperature rise or temperature fall, is acquired and stored in the storage circuit, as in the traditional technique, only the frequency temperature information in one direction is saved. Therefore, if temperature changes in the opposite direction to the direction in the case of acquiring the frequency temperature information, even though frequency correction is carried out in the system, a difference in oscillation frequency due to the hysteresis characteristic remains as a correction error. This correction error causes problems such as the need for a longer time for positioning, the consequent occurrence of a positioning error, and the risk of tuning failure with GPS satellites.